Faunal responses to oxygen gradients on the Pakistan margin: A comparison of foraminiferans, macrofauna and megafauna

diversity trends were similar among protists (foraminiferans and gromiids), metazoan macrofauna and megafauna along a transect (140–1850m water depth) across the OMZ during the 2003 intermonsoon (March–May) and late/post-monsoon (August–October) seasons. All groups exhibited some drop in abundance in the OMZ core (250–500m water depth; O2: 0.10–0.13mL L 1 1⁄4 4.46–5.80mM) but to differing degrees. Densities of foraminiferans 463mm were slightly depressed at 300m, peaked at 738m, and were much lower at deeper stations. Foraminiferans 4300mm were the overwhelmingly dominant macrofaunal organisms in the OMZ core. Macrofaunal metazoans reached maximum densities at 140m depth, with additional peaks at 850, 940 and 1850m where foraminiferans were less abundant. The polychaete Linopherus sp. was responsible for a macrofaunal biomass peak at 950m. Apart from large swimming animals (fish and natant decapods), metazoan megafauna were absent between 300 and 900m (O2 o0.14–0.15 mL L 1 1⁄4 6.25–6.69mM) but were represented by a huge, ophiuroid-dominated abundance peak at 1000m (O2 0.15–0.18mL L 1 1⁄4 6.69–8.03mM). Gromiid protists were confined largely to depths below 1150m (O2 40.2mL L 1 1⁄4 8.92mM). The progressively deeper abundance peaks for foraminiferans (463mm), Linopherus sp. and ophiuroids probably represent lower OMZ boundary edge effects and suggest a link between body size and tolerance of hypoxia. Macroand megafaunal organisms collected between 800 and 1100m were dominated by a succession of different taxa, indicating that the lower part of the OMZ is also a region of rapid faunal change. Species diversity was depressed in all groups in the OMZ core, but this was much more pronounced for macrofauna and megafauna than for foraminiferans. Oxygen levels strongly influenced the taxonomic composition of all faunal groups. Calcareous foraminiferans dominated the seasonally and permanently hypoxic sites (136–300m); agglutinated foraminiferans were relatively more abundant at deeper stations where oxygen concentrations were 40.13mL L ( 1⁄4 5.80mM). Polychaetes were the main macrofaunal taxon within the OMZ; calcareous macrofauna and megafauna (molluscs and echinoderms) were rare or absent where oxygen levels were lowest. The rarity of larger animals between 300 and 700m on the Pakistan Margin, compared with the abundant macrofauna in the OMZ core off Oman, is the most notable contrast between the two sides of the Arabian Sea. This difference probably reflects the slightly higher oxygen levels and better food quality on the western side. & 2008 Published by Elsevier Ltd. Elsevier Ltd. : +442380596247.